In the field of pathology, as an alternative to optical microscopes which are one of the tools of pathological diagnosis, virtual slide systems enable pathological diagnosis on a display through imaging and digitization of a sample that is placed on a slide. Digitization of a pathological diagnostic image using the virtual slide system makes it possible to handle, in the form of digital data, conventional optical microscopic images of samples. This is advantageous in terms of, for instance, faster remote diagnosis, briefing to patients using digital images, sharing of rare cases, and greater education and training efficiency.
In order to realize, through virtualization, the operation of an optical microscope in a virtual slide system, the entire image of a sample placed on a slide must be digitized. Through digitization of the entire image of the sample, the digital data generated by the virtual slide system can be observed using viewer software running on a PC or work station. Upon digitization of the entire image of the sample, the number of resulting pixels is enormous, ordinarily of several hundreds of millions to several billions, which translates into a huge volume of data.
Although the volume of data generated by the virtual slide system is enormous, images can be observed microscopically (detailed enlarged image) and macroscopically (whole overhead image), through enlargement or reduction in the viewer. This affords various benefits. Low-magnification images to high-magnification images can be instantaneously displayed, at the resolution and magnifications required by the user, through preliminary acquisition of all the necessary information items.
However, ruggedness in a cover glass, a slide glass and a specimen gives rise to waviness in the slide. Even in there is no such ruggedness, the specimen has a thickness of its own, and the depth position of tissue or cells to be observed depends on the observation position (in the horizontal direction) of the slide. Accordingly, configurations exist wherein a plurality of images is captured by varying the focal position along the optical axis direction, with respect to one slide (object). In such a configuration, acquired image data of the plurality of images acquired by virtue of such a configuration is referred to as a “Z stack image”, and plane images, at respective focal positions, that make up the Z stack image, are referred to as “layer images”.
In the virtual slide system, the specimen is ordinarily shot at each local region of the specimen at a high magnification (high NA), from the viewpoint of efficiency, and the shot images are spliced to generate thereby a complete image. The complete image has high spatial resolution, but shallow depth of field. In the virtual slide system, accordingly, the depth of field of a low-magnification image (for instance, objective lens 10×) resulting from reducing a high-magnification image (for instance, objective lens 40×), is shallower than the depth of field of an image directly observed in the optical microscope, and contains defocusing that was not present in the original low-magnification image. In pathological diagnosis, total screening must be performed in order to avoid overlooking lesion sites in low-magnification images. Therefore, the pathologist lays emphasis on the image quality of low-magnification images, and low-magnification images must be generated that exhibit little quality deterioration on account of defocusing.
The below-described conventional approaches have been proposed as regards enhancing image quality in low-magnification images in virtual slide systems. PTL1 discloses a configuration wherein layer images selected from among a Z stack image are displayed upon display of a high-magnification image; upon display of a low-magnification image, by contrast, an image totally in focus (all-in-focus image) is synthesized and displayed by using all the layer images that make up a Z stack image.
A below-described conventional technology relating to control of depth of field has been proposed. PTL2 discloses a configuration wherein coordinate conversion processing is performed on a plurality of shot images with varying focal position, in such a manner that the shot images match a three-dimensional convolution model, and the depth of field is extended as a result of three-dimensional filtering processing that involves modifying blurring on a three-dimensional frequency space.